An image sensor is a device for transforming first-order, second-order or more optical information such as optical images into electrical signals. An image sensor is generally classified into a complementary metal oxide semiconductor CMOS (Complementary Metal-Oxide Semiconductor) image sensor and a charge coupled device CCD (Charge Coupled Device) image sensor. CCD image sensors have excellent characteristics such as photosensitivity and anti-noise properties as compared to CMOS image sensors, however, highly integrated assembly of CCDs is difficult and CCDs require large quantities of electric power. On the other hand, CMOS image sensors, also called CISs, can be fabricated by more simple processes than those employed for CCD image sensors, are easily formed into highly integrated assemblies, and consume relatively little electric power.
Recent techniques for fabrication of CMOS image sensors and characteristics thereof have been greatly improved in conjunction with rapid technological advancement in the semiconductor industry, therefore, a great deal of research and investigations into development of CMOS image sensors are presently underway. The use of micro-lens (ML) for collection of external light may result in improvement in CIS resolution. In this regard, a photo-resist (PR) is typically used to manufacture an ML, but has a drawback of poor mechanical strength. This drawback has led to increases in the failure rate due to particles generated during chemical mechanical polishing (CMP).
In order to overcome the problems described above, low temperature oxides (LTO) with excellent mechanical strength have been applied to PRs. However, this also has problems in that cracks occur at an interface between the PR and the LTO and propagates through a chip, thereby causing damage to the entire chip during use of a semiconductor device. As a proposal to solve this problem, there is a method for fabricating the entire ML using an LTO whereby a reflowed PR is used as a mask to etch the LTO and, in turn, produce a lens. However, for fabrication of a lens shaped material such as a PR based ML, reflowing PR generates an uneven seed layer.
Example FIG. 1A illustrates a condition of an ML before PR reflowing that exhibits a favorable critical dimension (CD) between MLs and high uniformity in a wafer. Example FIG. 1B illustrates a condition of an ML after PR reflowing that exhibits significant deterioration of each CD between MLs and uniformity in a wafer. That is, there will occur a great difference in CDs between pixels after PR reflowing. Example FIG. 1C illustrates a cross-sectional view of a wafer after PR reflowing. Example FIG. 1D is a tilt image illustrating the condition of the wafer after PR reflowing.
Example FIGS. 2A to FIG. 2C illustrate a seed layer generated after etching an LTO using a reflowed PR as a mask. As illustrated in example FIGS. 2A to FIG. 2C, a surface of the seed layer is rough and shows poor uniformity after the etching process. More significant roughness effects can be observed near edges of pixels and line edge roughness is also serious. One reason behind the problems disclosed above is presumed to be because PR used for etching has a small thickness.
When depositing an LTO on and/or over a seed layer with high surface roughness (i.e., poor surface feature) as described above, an ML with poor surface roughness is formed as shown in example FIGS. 3A to FIG. 3C.
Accordingly, such processes for reflowing PR have disadvantages such as difficulty in statistical process control (SPC) and extreme roughness in edges patterned by a reflowed PR. Therefore, using the reflowed PR as a mask to etch an LTO still has a problem of poor edge profile. Such poor edge profile may induce a difference of characteristics between pixels in a microlens array, thereby causing a decrease in performance of a semiconductor device.